1. Field of the Invention
The invention relates to the manufacture of a tubular component that includes an insert made of a metal matrix composite.
2. Discussion of the Background
In the field of aeronautics in particular, one constant objective is to optimize the strength of components for a minimum mass and minimum size. Thus, certain components may from now on include an insert made of a metal matrix composite. Such a composite comprises a metal alloy matrix, for example a titanium (Ti) alloy, in which fibers extend, for example silicon carbide (SiC) ceramic fibers. Such fibers have a tensile strength very much greater than that of titanium (typically 4000 MPa compared with 1000 MPa). It is therefore the fibers that take the loads, the metal alloy matrix providing a function of binder with the rest of the component and also the function of protecting and isolating the fibers, which must not come into contact with one another. Furthermore, the ceramic fibers are resistant to erosion but necessarily have to be reinforced with metal.
These composites may be used in the manufacture of disks, shafts, ram bodies, casings, and spacers, such as reinforcements for monolithic components such as blades, etc.
To obtain such a composite insert, filaments called “coated filaments”, comprising a metal-coated ceramic fiber, are formed beforehand. The metal gives the filament the elasticity and the flexibility needed for handling it. Preferably, a very fine carbon or tungsten filament lies at the center of the fiber, along its axis, this carbon filament being coated with silicon carbide, while a thin film of carbon is interposed between the fiber and the metal, in order to provide a diffusion-barrier/buffer function during differential thermal relaxation that occurs as the liquid metal deposited on the fiber cools.
The manufacture of composite filaments, or coated filaments, may be carried out in various ways, for example by vapor deposition of metal in an electric field, by electrophoresis using a metal powder, or else by dip-coating ceramic fibers in a bath of liquid metal. Such a coating process, in which ceramic fibers are dipped into a liquid metal, is presented in patent EP 0 931 846 in the name of the Applicant. The manufacture of the filaments according to this process is rapid. Thus, composite filaments or coated filaments are obtained that serve as the basis for the manufacture of the composite insert which will be included in the component.
In the known processes for obtaining a component with an insert made of a metal alloy matrix composite, the coated filament is then formed from a workpiece called a preform. Such a preform is obtained by winding the coated filament between two metal retaining flanges that extend around a central mandrel. The winding is performed in a spiral, the preform obtained being in the form of a disk, the thickness of which is that of the coated filament constituting it. To ensure cohesion of the preform, the retaining flanges include apertures through which a material providing a bonding function, for example an acrylic resin, is sprayed.
FIG. 1 shows schematically one embodiment of a component with a composite insert. A plurality of preforms 1, each in the form of a disk, are stacked in a container 2 of cylindrical overall shape. The container has an annular cavity 3, the sectional shape of which, transverse to the axis 4 of the container, is that of the preforms 1. Preforms 1 are stacked until the entire height of the cavity 3 is filled. Typically, 80 preforms are thus stacked. This operation is manual.
It is then necessary to perform a binder-removal operation followed by degassing, so as to eliminate the binder, for example an acrylic resin, from the preforms 1. This is because no contaminating element must remain, when cold and hot, in contact with the titanium during the pressing stage.
An annular lid 5, having a projection 6 of shape complementary to that of the annular cavity, but of smaller axial dimension, is placed on top of the container 2, the projection 6 being brought into contact with the upper preform 1. The lid 5 is welded to the container 2, for example by electron beam welding, the assembly preferably being placed in a vacuum. There follows a step in which the assembly undergoes hot isostatic pressing. During this operation, the insert composed of juxtaposed coated filaments is compacted, the metal sheaths of the coated filaments being welded together and welded to the walls of the cavity 3 of the container 2 by diffusion, in order to form a dense assembly composed of the metal alloy (for example a titanium alloy) within which the ceramic fibers (for example SiC fibers) extend annularly.
A cylindrical component is obtained that includes an insert of a composite, resulting from the compaction of the stacked preforms 1. This component may optionally undergo a stress relaxation treatment, making it possible to compensate for the differential expansion between the ceramic fibers and the metal, in which they are embedded, when the assembly cools.
The component is then generally machined so as to obtain the final component.
This process for manufacturing a component with a composite insert has many drawbacks, and is difficult to exploit on an industrial scale owing to the length, complexity and precision required of its steps.
Firstly, since the ceramic fibers are brittle, the operations on the coated filaments must above all prevent any contact between them, and the welding of coated filaments has not been envisaged hitherto.
Furthermore, the binder-removal and degassing operations are not only lengthy, but there is never certainty that all of the binder has been removed. To ensure complete disappearance of the binder, necessary in particular for the correct subsequent behavior of the titanium alloy, several binder-removal and degassing steps are needed. This lengthens the total duration of the process and increases its overall cost.
In addition, should the filament break while it is being wound between the two flanges, it is necessary to form a new preform in so far as at the present time no means exist for solving the problem and resuming the winding.
Moreover, the step of positioning the coated filament preforms in the container is currently manual. The cost of the operation and in particular its precision are affected thereby. Now, the positioning of the coated filament in the container is a critical factor in the manufacturing sequence in so far as it determines the performance of the composite, with a very great influence of the orientation of the ceramic fiber according to the principal stresses of the component. It also determines the quality of the composite, by preserving the integrity of the ceramic fiber, during the various steps in the manufacture of the component. Lastly, it determines the final cost of the component, again because the operations of positioning the coated filaments are relatively lengthy and carried out manually. The positioning of the filaments in the container should therefore benefit from being improved.
The techniques of the prior art relating to the manufacture of components with an insert made of a composite propose the manufacture of annular components, such as rotor disks. It has been envisaged to manufacture tubular components, especially rotor shafts, with an insert made of a composite, but great technical difficulties have prevented the industrial implementation of such manufacture. A process has been envisaged which proposes to slip, into a sheath, mutually parallel coated filaments that extend longitudinally between two mandrels forming the sheath, and then subjecting the whole assembly to hot isostatic pressing. Inserting the coated filaments proves to be very tedious, if not impossible in the case of the latter filaments if it is desired to obtain a high density of coated filaments. Moreover, the risks of improperly positioning the filaments, which would damage them and result in the incorrect behavior of the component, are high. It has also been envisaged to stack preforms of coated filaments over the entire length of the sheath, but the drawbacks of such stacking were mentioned earlier. Moreover, it would be desirable for the ceramic fibers not to extend transversely to the component, for better uptake of the forces.